1. Field of the Invention
The present invention relates to dies for applying insulation enamel coatings to wires (hereinafter referred to as “coating dies”), and particularly to a die for coating wires for use in electrical equipment such as motors and transformers. Furthermore, the invention relates to methods for manufacturing enameled wires using the invented dies.
2. Description of Related Art
Enameled wires (enamel covered insulated wires) are widely used for coil wires in electrical equipment such as motors and transformers. Such enameled wires are formed by covering an insulation coating around a metal conductor having a desired cross section (e.g., circular and rectangular) depending on the application and shape of the coil. With the current trend toward small and high power vehicle motors (e.g., motors for electrical equipment and alternators), there is a requirement to reduce the thickness of insulation enamel coatings for wires so that such wires can be wound into coils at a higher filling factor. Also, there is another requirement for insulation coatings having a uniform thickness because an uneven thickness of insulation coatings can induce insulation breakdown due to concentration of electric fields.
In order to form a thin and uniform insulation coating on a wire, it is extremely important to position (center) the wire to be coated properly with respect to a coating die. Generally, in order to obtain an enameled wire having a coating of a predetermined thickness, an insulation varnish application and baking process is often repeated several times. As a result, an accurate centering procedure also needs to be repeated for a plurality of coating dies, which requires much labor. Meanwhile, whether the centering procedure is accurate or not is typically judged by observing a cross section of the resulting enameled wire after the varnish application and baking process.
One technique to center a wire to be coated with respect to a coating die is to utilize a pressure difference caused by the insulation varnish flow around the wire in the coating die (self-centering force). Since this self-centering force depends largely on various parameters (e.g., a wire feed rate, an insulation varnish viscosity, a gap between the coating die and the wire, a length and an angle of approach portion of the coating die, etc.), coating dies need to be optimally designed for different specifications of enameled wires. Therefore, with this technique, it is difficult to accommodate sudden changes in the specifications of enameled wires.
Meanwhile, JP-U Hei 7 (1995)-1539 A (Japanese Utility Model Application Publication) discloses a die for applying varnish to a core wire, including: a die body; and a die hole formed through the die body, the core wire to be passed through the die hole, in which the die hole has a core wire entry hole portion and successively a varnish restriction hole portion. The die further comprises a guide which aligns the core wire on the center axis of the varnish restriction hole portion. The guide is composed of three or more guide wires, or three or more protrusions provided at predetermined intervals in the circumferential direction on the inner surfaces of the core wire entry hole portion and the varnish restriction hole portion. According to JP-U Hei 7 (1995)-1539 A, since the guide composed of the guide wires or protrusions restricts the core wire passing position in the varnish restriction hole portion so that the core wire always passes along the center axis of the varnish restriction hole portion, the die for applying varnish is capable of applying a uniform varnish coating to the core wire even if the viscosity of the varnish to be applied is low.
However, even an enameled wire having an insulation coating formed by using such a die for applying varnish as disclosed in JP-U Hei 7 (1995)-1539 A can have regions involving an air bubble (air bubble regions) in the insulation coating. If the regions involving an air bubble (air bubble regions) are locally formed in the insulation coating of an enameled wire, an insulation breakdown is prone to occur. In addition, the air bubble regions adversely affect the electrical and mechanical properties of the enameled wire.
Therefore, it is desired that such air bubble regions do not exist in the insulation coating of an enameled wire. The formation of an air bubble region in an insulation coating is attributable to many factors. In many cases, however, a foreign matter such as a baking dross formed during a baking process or a half-peeled flaw remaining on a surface of the wire conductor can be an origin of the air bubble region in the subsequent varnish application process.
Herein, it is believed that half-peeled flaws originate mainly from streak flaws on a wire rod from which wire conductors are formed. Such streak flaws develop during wire rod manufacturing processes. Therefore, subjecting a wire rod to a peeling process is generally effective in reducing half-peeled flaws. However, in the case where cast defects are present in a wire rod, it is technically difficult to remove all the cast defects only by subjecting the wire rod to a peeling process. Also, cast defects that cannot be removed by a peeling process are prone to become exposed on a surface of a wire conductor as they are elongated during a wire drawing process, or they may exist barely covered by a thin layer of the conductor material. In the latter case, bending by a pulley or sliding with a gasket can cause such defects to appear on the surface and the thin layer covering such defects to curl up and become half-peeled flaws.